Excep&ons and file I/O

Transcription

1 Excep&ons and file I/O Exception in thread "main" java.lang.numberformatexception: For input string: "3.5" at java.lang.numberformatexception.forinputstring(numberformatexception.java:48) at java.lang.integer.parseint(integer.java:458) at java.lang.integer.parseint(integer.java:499) at AddNums.main(AddNums.java:8) CSCI 136: Fundamentals of Computer Science II Keith Vertanen Copyright 2013

2 Excep-ons Overview Handling unexpected events e.g. File is missing e.g. Trying to parse "$56.89" as a double File input/output Output to a text file Input from a text file Parsing the input using a Scanner Frees us from the limits of standard input Read from mul-ple files, read data twice, etc. 2

3 Adding two numbers Goal: Defend against all types of bad input Crashes if less than 2 arguments public class AddNums public static void main(string [] args) int num1 = Integer.parseInt(args[0]); int num2 = Integer.parseInt(args[1]); int sum = num1 + num2; System.out.println(num1 + " + " + num2 + " = " + sum); % java AddNums Exception in thread "main" java.lang.arrayindexoutofboundsexception: 0 at AddNums.main(AddNums.java:5) 3

4 Adding two numbers Goal: Defend against all types of bad input Take care and stay in bounds in array Crashes if passed a non- integer argument public class AddNums public static void main(string [] args) if (args.length < 2) System.out.println("AddNums <integer 1> <integer 2>"); return; int num1 = Integer.parseInt(args[0]); int num2 = Integer.parseInt(args[1]); int sum = num1 + num2; System.out.println(num1 + " + " + num2 + " = " + sum); % java AddNums Exception in thread "main" java.lang.numberformatexception: For input string: "3.5" at java.lang.numberformatexception.forinputstring(numberformatexception.java:48) at java.lang.integer.parseint(integer.java:458) at java.lang.integer.parseint(integer.java:499) at AddNums.main(AddNums.java:8) 4

5 Adding two numbers How to check for invalid inputs to parseint()? e.g. 1.0, , $1, 123., one public class AddNums public static void main(string [] args) if (args.length < 2) System.out.println("AddNums <integer 1> <integer 2>"); return; int num1 = Integer.parseInt(args[0]); int num2 = Integer.parseInt(args[1]); int sum = num1 + num2; System.out.println(num1 + " + " + num2 + " = " + sum); 5

6 Java excep-ons When things go wrong: Java "throws" an excep-on An excep-on is an object that can be caught Programmer can decide if the program can recover or not Rather than crashing with a run-me error 6

7 try- catch block try // Do some risky things // Do some more risk things catch (Exception ex) // Try and recover from the problem The mother of all excep-on types. All other types of excep-ons inherit from this parent class. If something goes horribly wrong on a line in the try block, flow of control immediately jumps to the catch block. Details about the excep-on are passed as an object into the catch block. Like a parameter list of a method, ex is just a name for the Exception object in the catch block. 7

8 Add code to catch all excep-ons public class AddNums public static void main(string [] args) try int num1 = Integer.parseInt(args[0]); int num2 = Integer.parseInt(args[1]); int sum = num1 + num2; System.out.println(num1 + " + " + num2 + " = " + sum); catch (Exception ex) System.out.println("Something went wrong!"); System.out.println("End of program"); % java AddNums Something went wrong! End of program % java AddNums Something went wrong! End of program Not an ideal solu-on: How is the user suppose to know what to do differently? 8

9 A beber solu-on public static void main(string [] args) if (args.length < 2) System.out.println("AddNums <integer 1> <integer 2>"); return; int num1, num2; try num1 = Integer.parseInt(args[0]); catch (NumberFormatException ex) System.out.println("1st argument invalid: " + args[0]); return; try num2 = Integer.parseInt(args[1]); catch (NumberFormatException ex) System.out.println("2nd argument invalid: " + args[1]); return; int sum = num1 + num2; System.out.println(num1 + " + " + num2 + " = " + sum); Principle 1: Don't catch stuff you can handle with appropriate program logic (such as staying in bounds in an array). Normal scoping rules apply inside a try- block, so ogen you'll need to declare variables outside if you need them ager the try- block. Principle 2: Don't use generic Excep-on class. Catch the specific type of excep-on you had in mind. 9

10 Wri-ng to a text file Java has many built in file I/O classes PrintWriter, class that allows wri-ng text to a file 10

11 Perfect square writer Goal: Write perfect squares to a file import java.io.*; So Java can find the PrintWriter class. public class PerfectSquareWriter public static void main(string [] args) PrintWriter writer = new PrintWriter("squares.txt"); for (int i = 0; i < 1000; i++) writer.println(i * i); writer.close(); PrintWriter constructor takes a string specifying the filename to write to. % javac PerfectSquareWriter.java PerfectSquareWriter.java:8: unreported exception java.io.filenotfoundexception; must be caught or declared to be thrown PrintWriter writer = new PrintWriter("squares.txt"); ^ 1 error 11

12 Perfect square writer Lesson: Some risky behaviors require try- catch import java.io.*; public class PerfectSquareWriter public static void main(string [] args) try PrintWriter writer = new PrintWriter("squares.txt"); for (int i = 0; i < 1000; i++) writer.println(i * i); writer.close(); catch (FileNotFoundException ex) System.out.println("Failed to open file!"); This line had to be in a try- catch block catching a FileNotFoundException (or a parent thereof). Not required, but good style to cleanup ager you are done with a file. % more squares.txt

13 Reading a text file Need two Java classes: File class, represents a filename System independent abstrac-on of a file's path Not actually used for reading or wri-ng Scanner class, parses out values Very similar to StdIn.read* methods 13

14 Handy methods: File class Method boolean canread() boolean canwrite() boolean delete() boolean exists() long length() String getname() String getpath() boolean mkdir() Descrip&on Test if the program can read from the file. Test if the program can write to the file. Abempt to delete the file. See if the given file exists. Get length of the file in bytes Returns the filename por-on of the path Returns the path without the filename Creates a directory specified by the path File file = new File("c:\\workspace\\nums.txt"); System.out.println(file.getName()); System.out.println(file.getPath()); Windows paths need escaping of the backslash character by using two of them. nums.txt c:\workspace\nums.txt 14

15 Handy methods: Scanner class Method String next() String nextline() int nextint() double nextdouble() Descrip&on Returns the next string, separated via whitespace Returns the en-re line up to the next line break Returns the next integer Returns the next double boolean hasnext() boolean hasnextline() boolean hasnextint() boolean hasnextdouble() Are there any more tokens available? Is there another line available? Can the next token be interpreted as an int? Can the next token be interpreted as a double? void close() Free up resources 15

16 public class AvgNums public static void main(string [] args) double sum = 0.0; long count = 0; while (!StdIn.isEmpty()) sum += StdIn.readDouble(); count++; System.out.println(sum / count); Original program, reads numbers from standard input. % java AvgNums < nums.txt Averaging numbers public class AvgNumsFile % java AvgNumsFile nums.txt public static void main(string [] args) double sum = 0.0; long count = 0; if (args.length < 1) System.out.println("AvgNumsFile <filename>"); return; try Scanner scanner = new Scanner(new File(args[0])); while (scanner.hasnext()) sum += scanner.nextdouble(); count++; scanner.close(); System.out.println(sum / count); catch (FileNotFoundException ex) System.out.println("Failed to open file!"); New program, read from filename given by args[0]. 16

17 Trying to break it public class AvgNumsFile public static void main(string [] args) double sum = 0.0; long count = 0; if (args.length < 1) % java AvgNumsFile noexist.txt Failed to open file! % java AvgNumsFile mobydick.txt Exception in thread "main" java.util.inputmismatchexception at java.util.scanner.throwfor(scanner.java:840) System.out.println("AvgNumsFile <filename>"); at java.util.scanner.next(scanner.java:1461) return; at java.util.scanner.nextdouble(scanner.java:2387) try at AvgNumsFile.main(AvgNumsFile.java:21) Scanner scanner = new Scanner(new File(args[0])); while (scanner.hasnext()) sum += scanner.nextdouble(); count++; scanner.close(); System.out.println(sum / count); catch (FileNotFoundException ex) System.out.println("Failed to open file!"); 17

18 Mul-ple catch blocks... double sum = 0.0; long count = 0; if (args.length < 1) System.out.println("AvgNumsFile <filename>"); return; try Scanner scanner = new Scanner(new File(args[0])); while (scanner.hasnext()) sum += scanner.nextdouble(); % java AvgNumsFile mobydick.txt count++; Invalid data in file! scanner.close(); System.out.println(sum / count); catch (FileNotFoundException ex) System.out.println("Failed to open file!"); catch (InputMismatchException ex) System.out.println("Invalid data in file!");... 18

19 Throwing excep-ons How do excep-ons start their life? Somebody "throws" them throw new NoCaffeineException(); Whoever is using the method has to catch Or else that method can throw it What if nobody catches it? Checked excep-ons Causes compile error, excep-ons that must be caught Methods marked with a "throw" clause Unchecked excep-ons Causes run-me error, excep-ons where catching is op-onal Usually a programming error e.g. ArrayIndexOutOfBoundsExcep-on, NullPointerExcep-on 19

20 Java excep-on class hierarchy hbp:// 20

21 Ducking an excep-on public static double getfileavg(string filename) throws FileNotFoundException double sum = 0.0; long count = 0; Scanner scanner = new Scanner(new File(filename)); while (scanner.hasnext()) sum += scanner.nextdouble(); count++; scanner.close(); return (sum / count); Not our problem anymore, whoever calls getfileavg() has to catch now. Methods can throw 0 or more excep-on types. public static void main(string [] args) if (args.length < 1) System.out.println("AvgNumsFile <filename>"); return; System.out.println(getFileAvg(args[0])); % javac AvgNumsFile.java AvgNumsFile.java:26: unreported exception java.io.filenotfoundexception; must be caught or declared to be thrown double avg = getfileavg(args[0]); ^ 1 error 21

22 Finally, the finally block Finally block executes no maber what If no excep-on, runs ager try- block If excep-on occurs, runs ager catch- block Useful for doing cleanup that is always needed try turnovenon(); x.bake(); catch (BakingException ex) ex.printstacktrace(); finally turnovenoff(); Prints out the stack trace (like what you see when you get a run-me error). Handy for debugging what line in the try- block is causing the trouble. 22

23 Excep-ons Summary An important part of wri-ng defensive code Many of Java classes require handling excep-ons e.g. File I/O, network communica-on, playing sounds, using threads Checked versus unchecked excep-ons File I/O Text file input/output PrintWriter class, wri-ng text to file Scanner and File classes, reading and parsing text 23